Keypad is the most important input interface of the personal computer and the laptop. Users instruct computer to execute the corresponding action by pressing different keys. Generally, according to the structure of the key of the keypad, there are two kinds of keypad, mechanical and film. The film keypad has the advantage of thin, slight, low noise, and low cost. Thus, the film keypad replaces the mechanical keypad and becomes more popular in the market.
Two detection methodologies to detect the key that user press are scan detection and interrupt detection respectively. The scan detection methodology exploits matrix topology to achieve the multi-key input purpose. The keypad manufacture has more flexibility when design the key of the keypad which has the larger amount of the key. So that most of film keypads in the market are using the scan detection methodology. In the conventional scan detection methodology, pull-up scan is the most common method in this field. FIG. 1a illustrates the conventional keypad circuit structure of the pull-up scan. The film keypad 10 in FIG. 1a has a matrix which crisscross-formed by a plurality of column signal lines 12 with a plurality of row signal lines 13. Each cross 14 formed by the column signal lines 12 and the row signal lines 13 has a switch S1, S2, S3, and S4, respectively. One terminal of each row signal lines 13 is electrically coupled to a micro controller unit (MCU) 11, and one terminal of each column signal 12 is electrically coupled to ground. FIG. 1b illustrates the simplified keypad circuit structure shown in FIG. 1a of the pull-up scan. As shown in FIG. 1b, the circuit in the keypad 10 just uses one switch S1 as an example for illustration purpose. In addition, column resistor Rcol and row resistor Rrow are used to indicate the column signal line and the row signal line respectively.
When the key corresponding to the switch S1 is pressed by the user, the switch S1 is turned on. To cooperate with the circuitry in the MCU 11 which includes a voltage source Vdd and a pull up resistor Rpu, the voltage Vio divided by column resistor Rcol and the row resistor Rrow can be detected according to the equation (1):Vio=Vdd*(Rcol+Rrow)/(Rpu+Rcol+Rrow)  (1)
When the film material of the film keypad 10 is silver paste membrane which the resistance value is just about couple hundred ohms and the resistance of the pull up resistor Rpu is around couple kilo-ohms, the voltage level of Vio is quite closed to zero when the switch S1 is turned on. As a result, the voltage level of Vio is lower than the standard of the Schmitt trigger, wherein the predetermined trigger voltage of the Schmitt trigger is 0.2˜0.4 times of the voltage source. Thus, it can scan all switches in the circuit of the keypad correctly. However, if the film material changes to carbon, the resistor resistance of the column resistor Rcol and the row resistor Rrow raise to couple hundred kilo-ohms, so that have to increase the resistance of the pull up resistor Rpu and/or the standard of the Schmitt trigger.
The material of the film is limited during the mass production due to the requirement of the resistance value of the pull up resistor Rpu and the standard of the Schmitt trigger, thus, the manufacturing need to adjust the resistor resistance of pull up resistor Rpu and/or the standard of Schmitt trigger accordingly.